This invention relates to improvements in measuring the concentration of suspended substances in a liquid, the substances consisting of components of different geometrical size. The liquid is arranged to flow and is illuminated transverse to its flow direction by one or more light sources. The radiation emitted from the liquid is detected.
Especially in the wood-processing industry it is of the greatest interest to be able to measure the concentration of suspended substances, mainly fibers and fiber fragments, in waste water of different types. According to standard methods now in use suspended substances means such materials as can be removed mechanically by means of filtration. These suspended substances may consist of many different components, mainly fiber materials, as well as different fillers from paper manufacture, which may vary considerably in size.
The instruments now available for measuring suspended substances, such as turbidimeters, are based on the general light scattering ability of the sample. Measuring means operating with polarized light permit determination of the concentration of suspended substances provided variations of composition do not occur. However, the existence of such variations is typical of waste water systems in the wood-processing industry, particularly during so-called occasional discharges, as a consequence of disturbance or functional trouble in the manufacturing process.
Instruments for measuring turbidity are often considerably more sensitive to discharges of a fine fraction (fiber fragments) than to discharges of first-rate fibers, and therefore an increase of the output signal from an instrument of this type may mean a big discharge of first-rate fibers or else a considerably less discharge of a fine fraction.
As fibers have a great value it is of interest to recycle fibers in waste water to production. If these fibers leave the external purification plants of the manufacturer, they may collect in fiber banks near to the discharge. On the other hand, the fine fraction may be suspended in the water for a long time and may cause an environmental disturbance of another kind. An increased number of first-rate fibers in a drain is often an indication that something in the system has gone wrong and must be corrected. Therefore it is often of interest to measure selectively the amount of first-rate fibers in a discharge. There is an instrument on the market, which substantially only reacts to the amount of first-rate fibers in a discharge, but is not sensitive in any degree worth mentioning to the amount of fine fraction. One conclusion is that there is no instrument today that can measure continuously the concentration of suspended substances in the connection with wood-processing industry in a sufficiently reliable way, if no special conditions are present.
The guiding principles of today for control of activity dangerous to the environment in the forest industry indicate that the concentration of suspended substances delivered by a manufacturer to a recipient stream or other body of water should be measured and reported to the authorities. The sharpened demands on control have also stressed the need of continuously recording instruments that can discover rapid fluctuations of the concentration level.
So far measurements of suspended substances in the forest industry have been carried out in the laboratories of the manufacturers by means of mechanical separation of a sample. The sample usually is obtained by means of an automatic sampler collecting samples at regular intervals for comparison to a standard sample, usually daily. This method is slow and relatively expensive, as it requires manual handling, and it will provide different results for different compositions of the sample. The advantage of the method is that it functions and is simple. The exactness is satisfactory, but care must be taken to maintain it. However, the method is only suitable for control of random samples.